Satellite positioning capabilities are being added to a wide variety of consumer, enterprise and safety products such as Personal Navigation Devices (PND), cell phones, asset tracking modules and in-vehicle systems to name a few. While most devices currently use GPS, the United States GNSS, several other GNSS are either operational or planned, including Russia's GLONASS and Europe's Galileo. Over the past decade, assistance techniques have emerged to optimize the performance of GPS and GNSS in mobile devices, specifically in terms of reduced TTFF, reduced power consumption and increased sensitivity. These are generally referred to as Assisted-GPS (AGPS) or more generically, Assisted-GNSS (AGNSS), where a server element is designed to deliver GNSS assistance data to a client device allowing the GNSS receiver in the client to acquire GNSS satellite signals more quickly and thus determine its location more effectively than if un-assisted.
The assistance data served to a requesting client device can contain several GNSS elements depending on the GNSS system in use and on what the client device's onboard GNSS receiver may already have in memory. The largest and most important assistance element is known as the Ephemeris. GNSS receivers require the orbit positions of the GNSS satellites at the time ranging signals are transmitted in order to compute a position solution. Currently, this orbit information is provided by the satellites on a radio frequency (RF) data link in the form of a satellite position model. The model utilizes a set of orbital elements, the Ephemeris, which in the case of GPS, is valid for a limited time period, typically 4 to 6 hours.
The GPS satellites broadcast the Ephemeris data on an RF data link, and the GPS receiver continuously monitors and demodulates this data stream to obtain updated Broadcast Ephemeris. The Ephemeris data is a mathematical orbit arc model that allows the user to evaluate a set of equations, and obtain the satellite position at any time during the model fit period. Although the model allows the evaluation of the satellite position beyond the 4 to 6 hours of validity, the accuracy typically degrades to the level of a kilometer within a day. For a more detailed description of GPS and the ephemeris model, see “Global Positioning System: Theory and Applications” edited by Parkinson and Spilker, Vol. 1, chapters 2 (signal structure), 4 (ephemeris model), and 9 (navigation solutions).
In AGNSS deployments with a large number of client devices, such as in mobile networks where AGPS has been implemented, the traffic load on the assistance server peaks around the expiry times of the Ephemeris data. For GPS, this is typically every two hours when devices simultaneously start to request fresh Ephemeris assistance data as the validity period of the previously received Ephemeris expires. As millions of devices may exist within a given AGNSS system, this cyclical peak traffic condition can result in congestion issues. These issues are currently typically mitigated by designing the server capacity around peak levels, adding excess capacity between cycles and increasing systems costs.
There is prior art in the form of patents, patent applications and articles attesting to the value of providing satellite orbits for longer time periods when the fresh Ephemeris is not readily available directly from the satellites or through an AGNSS server. These are generally known as Extended Ephemeris techniques.
In McMahan et al., (“Position detection system integrated into mobile terminal”, U.S. Pat. No. 6,437,735, Aug. 20, 2002), almanac data is created in the memory of the mobile terminal by converting the ephemeris information into almanac information. The stored almanac information is then used for faster satellite signal acquisition and position calculation at a later time. The Ephemeris information may be received from either the satellites or an AGNSS server.
A method in Garin et al. (Garin et al., “Determining position without current broadcast ephemeris”, Patent Application No: US 2008/0129593 A1, Jun. 5, 2008), determines the satellite orbits without broadcast Ephemeris data for extended time periods. The mobile device stores the satellite sates extracted from the broadcast Ephemeris and numerically integrates equations of motion with regard to the already stored satellite states to determine current satellite states. The satellite state predictor determines current satellite states using appropriate force models which include Earth gravity model, luni-solar model, solar radiation pressure and yaw-bias parameters.
Finally, Han et al. (“Method and apparatus in standalone positioning without broadcast ephemeris”, Patent Application No: US 2008/0111738 A1, May 15, 2008) discloses methods for enabling a standalone navigation device to generate predicted satellite orbits based on historical information stored in the device without the need of connecting to a remote assistance data server. The predicted orbits may be accurate for several days without reception of broadcast Ephemeris.